1. Field of the Invention
The present invention relates to a method and an apparatus for the manufacture of printing masters by electrophotographic means, said apparatus comprising a charging means for charging the printing masters prior to exposure, a lighting means for illuminating the original, an imaging means for projecting the image of the original onto the charged printing form, and transportation, development, and drying means; said method comprising arranging an original in a holder, mechanically transferring the unexposed printing master by reduced pressure from a printing master holder to an exposure platform electrostatically charging the unexposed printing master, exposing the printing master to an image of the original projected through an imaging means by way of an illuminating means, mechanically transferring the exposed printing master to a developing station, developing and fixing the latent electrostatic image on the printing master, mechanically transferring the developed printing master to a drying station, drying the developed printing master, and mechanically discharging the finished printing master from the apparatus.
2. Description of the Prior Art
The liquid development of printing masters exposed to an image by emmersion, roller application, spraying, and/or passage through a developer bath, followed by drying by heat action, is known. As far as these process steps are manually performed, they involve the risk that the printing form is not always treated with the necessary uniformity, so that the final fixed image on the printing form may show irregularities.
The expression "printing master" as used in this specification shall also encompass printing plates and the like.
If any of the known apparatuses is used which allow a rapid and uniform operation, the light-sensitive layer carrying a latent electrostatic image corresponding to the optical image of the original is repeatedly contacted with the transportation means as it is conveyed from one treating station to the next, so that undesirable alterations in and destructions of the light-sensitive layer may occur, for example, by dust particles deposited on the means of transportation. Further, undesirable alterations, occurring, for example, when the charge of an electrophotographic printing master varies, may be caused by contact alone.
In one known attempt to eliminate these disadvantages, a developing device for printing masters has been proposed which comprises an obliquely arranged support on which the printing master is placed. The support is associated with a developer nozzle and a blast nozzle, and the support and the nozzles are so arranged that they can be moved relative to each other. This developing arrangement is only one of several stations used for the manufacture of electrophotographic printing masters, which, as far as the development process is concerned, yield faultless printing masters suitable for technical purposes, but do not influence the undesirable alterations in and destructions of the light-sensitive layer caused by the other process steps necessary in the production of the electrophotographic printing masters.
In another known device, which is associated with a copying apparatus, the image of an original is projected onto the printing master and the printing master is then converted into a printing surface from which copies may be printed in a conventional printing apparatus. The copying apparatus comprises an exposure and activating station including a developing unit in order to develop an ink-receptive image on the surface of the printing master. For this purpose, the printing master is exposed in the exposure station under an image of the original with which it is in contact, i.e., light is passed through the graphic original onto the light-sensitive printing master. The image of the original forms a shadow image in the corresponding areas of the printing master, thus discharging the printing master is a differentiated pattern and creating a latent, electrostatic image. The exposure device comprises, inter alia, a transparent cylinder surrounding a coaxially arranged light source. By conveying the printing master from one treating station to the next, the light-sensitive layer carrying the latent, electrostatic image of the original is constantly contacted with the transportation means, so that the above-mentioned undesirable alteration and destruction of the light-sensitive layer, for example, by dust particles deposited on the conveying system, may occur. Moreover, the exposure device of this known apparatus does not permit an edge-free illumination of the orignial, but the edges of the original are always reproduced on the printing master as fine shadow images.
Further, the known apparatuses have the drawback that they do not permit printing masters of varying sizes to be automatically fed without damaging or soiling them, and that it is impossible to light the printing masters without the formation of shadows or expose them in such a manner that the scale of reproduction is linearly changed in one direction.